Breast cancer is the most common cancer and the second leading cause of cancer deaths among women. The majority of patients present with operable disease. In fact, most patients with node-negative disease will be cured by surgical resection of their tumor alone. However, due to proven efficacy, most patients with invasive breast cancer will undergo a complete axillary dissection for lymph node evaluation, and are considered candidates for adjuvant (systemic) therapy. This subjects the majority of patients with node-negative disease to the expense and side effects of axillary dissection and systemic chemotherapy. Primary treatment failure is secondary to undetectable spread of tumor, and this occult tumor is the target of adjuvant therapy. He has developed extremely sensitive methods for the detection of occult metastases in bone marrow (BM) and regional lymph nodes in patients with breast cancer. Studies from their laboratory and others strongly suggest that the presence of occult regional and systemic metastases are associated with increased recurrence and decreased survival. Furthermore, he has shown that the identification and analysis of the sentinel lymph node (SLN, the first lymph node draining the tumor bed) may reduce the need for complete axillary dissection, particularly in patients with node-negative disease. Their central hypothesis is that the presence of occult metastases to BM and SLN in patients with early stage breast cancer identifies patients at increased risk of recurrence and death, and identifies the specific target of adjuvant chemotherapy. The specific aims are: (1) To identify and quantitate occult metastases in BM and SLN of patients with breast cancer. This study will be organized by the NIH/NCI funded American College of Surgeons Oncology Group (ACOS-OG), (protocol Z0010). (2) Because they understand the difficulties of current methods to detect occult metastases, he has developed and will evaluate novel imaging and molecular methods for the detection of occult metastases. (3) To evaluate, through the ACOS-OG data management and follow up capabilities, the clinical significant of occult metastases to BM and SLN. This study will provide the essential correlative research component for the ACOS-OG multi-center clinical trial Z0010, and should provide definitive evidence regarding the use of occult metastasis detection in patients with breast cancer. The detection of occult metastases in BM and SLN, in place of more radical axillary dissection, could allow for more specific yet less invasive assessment of patient risk, and redefine the surgical and systemic treatment options for patients with breast cancer.